The chemical elements that make up the world, according to the Association.
Wind, earth, water, fire, void (ether).
Water, fire, ground, wood, metal.
A magus is normally going to bear one of these. Fire is called normal, wind is called noble.
However, there are those that bear elements that belong to neither of these, but they tend to not be part of the Association or belong to very specialized houses. Alignment can be altered, but it is extremely, extremely, dangerous and painful.
It is possible to have more than one element as well as compound elements. Often times, magi having multiple elements will work with the common relationships between their elements to master specific categories of spells (for instance, "liquid manipulation" via the water and wind elements). Someone who has all five elements are called an Average one.